Network Troubleshooting Guide

When something goes wrong on the network, a systematic approach saves hours. Here's a practical guide to diagnosing common issues.

Step 1: Check Physical/Basic Connectivity

• Is the cable plugged in? Is WiFi connected?
• Can you ping your default gateway? (ping 192.168.1.1)
• Can you ping a public IP? (ping 8.8.8.8)

Step 2: DNS Resolution

If you can ping 8.8.8.8 but not google.com, it's a DNS issue:

• nslookup google.com — does it resolve?
• Try a different DNS server: nslookup google.com 8.8.8.8
• Flush DNS cache: ipconfig /flushdns (Windows) or sudo dscacheutil -flushcache (macOS)

Step 3: Trace the Path

traceroute (or tracert on Windows) shows every hop between you and the destination. Look for:

• High latency jumps (indicates congestion)
• Timeouts at specific hops (firewall or routing issue)
• Loops (misconfigured routing)

Step 4: Check Ports and Services

• netstat -an — see all active connections and listening ports
• telnet host 80 or nc -zv host 443 — test if a specific port is reachable
• curl -v https://example.com — detailed HTTP connection info

Step 5: Packet Capture

When all else fails, tcpdump or Wireshark captures actual network packets for deep analysis. Filter by host, port, or protocol to find exactly what's happening on the wire.

Common Issues Quick Reference

• "No internet": Check gateway → check DNS → check ISP
• Slow browsing: Speed test → traceroute → DNS response time
• "Connection refused": Service not running or firewall blocking the port
• "Timeout": Network unreachable, firewall drop, or server down